What is the fitness outcome of crossing unrelated fish populations? A meta-analysis and an evaluation of future research directions

Abstract

Outbreeding has been shown to decrease fitness in a variety of species, including several species of fish. An understanding of the general outcomes following outbreeding is required in order to understand the consequences of conservation-related actions and hybridization in aquaculture. A meta-analysis was conducted on outbreeding studies in fishes using 670 comparisons between parent populations and their hybrid progeny. Five hundred and seventy-six comparisons involved first generation hybrids (F1), and a much smaller number (94) involved second generation hybrids (F2). The overall response to outbreeding in the F1 and F2 generations was positive and significant (F1: d i+ * = 0.425 ± 0.121; F2: d i+ * = 0.548 ± 0.295, where d i+ * is the effect size of studies within generations); however, responses differed when studies were separated into groups describing the experimental environment, taxon, or trait. Findings may be biased by a few studies of large effect. Genetic distance explained little of the variance in effect sizes across studies. Results of the meta-analysis suggest that there is no reliable predictor for the effects of outbreeding in fishes, although inconsistencies in experimental design were noted across studies. Future research should include comparisons in both parental environments in order to detect the underlying mechanisms of outbreeding depression, and should focus on measurement of equivalent sets of fitness-related traits. Experimental design should permit estimation of genetic distances based on quantitative traits, which may in turn be meaningful predictors of the outcomes of outbreeding depression. Concerted and consistent research in this area will provide information of relevance to conservation, aquaculture and evolutionary studies.

Effects of genotypic and phenotypic variation on establishment are important for conservation, invasion, and infection biology

“…That increased genotypic and phenotypic diversity offers a means to increase establishment success illustrates that the role of individual variation deserves increased consideration by designers, managers, and practitioners involved in conservation biology (3,38). In this context, it should be emphasized that mixing individuals from different populations to increase diversity can actually reduce population fitness (39). It is particularly promising that diversity seems to contribute to successful establishment most strongly in experiments performed under natural conditions in the wild, because in those studies the fate of the introduced individuals and their progeny may be influenced by a plethora of abiotic factors and biotic interactions beyond the control of the experimenter, as in reintroduction and translocation programs.…”

“…The rationale for excluding such experiments is that genetic admixture resulting from interbreeding between individuals from different populations may have either positive or negative effects on population fitness depending on the evolutionary history, genetic architecture, and environmental contexts of the populations involved, rather than on the degree of genetic and phenotypic variation per se (1,39,57,58). This is because even though immigration may alleviate the effects of inbreeding depression, it may also disrupt local adaptations and result in the breakup of coadapted gene complexes and favorable trait-value combinations (i.e., outbreeding depression) (23,73).…”

Genetic and phenotypic changes in an Atlantic salmon population supplemented with non-local individuals: a longitudinal study over 21 years

“…These results may suggest some outbreeding depression, a hypothesis that would require an experimental approach to be validated. Outbreeding depression has been frequently described in numerous taxa [10] including salmonids ( [27,28,51,57,58], review in [29]). …”

Variation in founder groups promotes establishment success in the wild

“…Founder groups were not introduced outside of the natural distribution range, and we therefore cannot discard the possibility that individuals may have immigrated into the experimental areas after the introductions. The influence of any such immigration is difficult to envisage because genetic admixture resulting from interbreeding between individuals from different populations may have either positive or negative effects [6,42,43].…”